Subsea Seawater Filtration Apparatus

ABSTRACT

A filtration apparatus includes a tubular casing having a longitudinal axis and first and second casing ends, a plurality of partition plates positioned in the casing and sealed thereto to thereby define a plurality of axially successive chambers within the casing, including an intake collection chamber between a first of the partition plates and the first casing end, a discharge collection chamber between a second of the partition plates and the second casing end, and a reject collection chamber opposite the second partition plate from the second casing end. A plurality of elongated filtration membrane stacks are positioned side-by-side in the casing generally parallel to the longitudinal axis. Each filtration membrane stack includes an intake end which is fluidly connected to the intake collection chamber, a discharge end which is fluidly connected to the reject collection chamber, and a permeate channel which extends between the intake and discharge ends and is fluidly connected to the discharge collection chamber, an end of the permeate channel located adjacent the intake end being sealed from the intake collection chamber. The filtration apparatus also includes an intake pipe having a first end fluidly connected to the intake collection chamber and a second end fluidly connected to a first connector located proximate the second casing end; a discharge pipe having a first end fluidly connected to the discharge collection chamber and a second end fluidly connected to a second connector located proximate the first connector; and a reject pipe having a first end fluidly connected to the reject collection chamber and a second end fluidly connected to a third connector located proximate the first and second connectors. Each filtration membrane stack includes a plurality of filtration membranes, and the plurality of filtration membrane stacks together define a plurality of axially successive sets of radially adjacent filtration membranes. Also, each filtration membrane of each of the sets of filtration membranes is sealed to a corresponding hole in a respective one of the partition plates.

The present application is a continuation of U.S. patent applicationSer. No. 16/619,072 filed on Dec. 3, 2019, which is a U.S. nationalstage filing of International Patent Application No. PCT/US2017/038752filed on Jun. 2, 2017.

The present disclosure relates to a seawater filtration apparatus whichcomprises multiple filtration membrane stacks. In one embodiment, thedisclosure is directed to a seawater filtration apparatus whichcomprises multiple filtration membrane stacks positioned in a singlevertical casing through which the intake, reject and discharge pipes arealso routed. The present disclosure is also directed to an improvedfiltration membrane and a filtration system which comprises such amembrane.

BACKGROUND OF THE INVENTION

Filtration assemblies are commonly used in the subsea hydrocarbonproduction industry to filter certain minerals from raw seawater priorto injecting the seawater into the hydrocarbon formation for artificiallift applications. Such filtration assemblies are typically locatedtopside (i.e., on a production vessel or platform) and usually includemultiple filtration membrane stacks which are each housed in acorresponding horizontally oriented casings. Large numbers of thehorizontal casings, e.g., up to sixty or more, are stacked together in asupporting frame, and the individual casings in each stack are connectedto a common collector by an external piping assembly. In thisarrangement, the large stacks of horizontally oriented casings and theexternal piping assembly required to connect the casings to thecollector contribute to a filtration assembly which is relativelycomplex and heavy and comprises a relatively large footprint.Consequently, such filtration system are impractical for use subsea.

Large footprint/weight requirements and high operating costs associatedwith intervention, cleaning and replacement are currently the keylimiting factors to the use of nano-filtration (NF) membranes inseawater filtration systems, especially in offshore (both topside andsubsea) facilities. As shown in the prior art seawater treatment systemdepicted in FIG. 4 , the seawater treatment process typically involvespre-treatment, nano-filtration and post-treatment (e.g., de-aeration)systems. The pre-treatment system usually comprises a coarse filter(e.g., screen filter, strainer) followed by a granular media filter or alow pressure-driven membrane such as a microfiltration (MF) orultrafiltration (UF) membrane. A biocide solution, typically chlorinecontaining an oxidizing solution, is injected into the pre-treatmentsystem or into the effluent of the pre-treatment system to control thegrowth of microorganisms in the NF system. NF membranes such as thosewith a polyamide active layer often degrade when exposed to greater than0.1 ppm of chlorine, and any residual chlorine should therefore bedeactivated by injecting a chemical chlorine scavenger or passing theliquid through activated carbon before feeding the pretreated water tothe NF system. The NF system comprises a number of uncoated membraneelements installed in multiple arrays of one or two stages. Typicalwater recovery of the NF system is 50% for a single stage and 75% fortwo stages. The NF system occupies around 50% of the overall footprintof the water filtration system.

A majority of NF membrane elements currently employed in waterfiltration systems installed in oil and gas fields comprise polyamidethin film composite membranes packaged in a spiral wound configuration.These membranes have the tendency to suffer from severe fouling due tothe accumulation of particulate/colloidal, organic and/or biologicalmatter, resulting in reduced water productivity and/or high operationalcosts (e.g., for chemical cleaning or replacement). When performance isreduced to below the pre-defined criteria (e.g., normalized permeateflow, salt passage or pressure drop), the filtration system (or a partof the system) is taken offline and cleaning-in-place (CIP) is performedwith a suite of chemicals from the chemical cleaning system. An exampleof one such cleaning procedure is described in “Dow Filmtec™Membranes—Cleaning Procedures for DOW FILMTEC FT30 Elements” publishedby the DOW Chemical Company. The combination of reduced membranepermeability, frequent downtime and high chemical consumption due tofouling translates directly to lower productivity and high operationalexpenditures (OPEX). Successful implementation of a filtration system inremote offshore topsides or even subsea facilities is largely dependenton the logistics of chemicals.

Therefore, a need exists for a reduced weight, small footprint waterfiltration system and/or a NF membrane which exhibits enhancedpermeability, selectivity, chlorine tolerance and protection fromfoulant accumulation.

SUMMARY OF THE INVENTION

In accordance with the present disclosure, a filtration apparatus isprovided which comprises a tubular casing having a longitudinal axis andfirst and second ends, a plurality of elongated filtration membranestacks positioned side-by-side in the casing generally parallel to thelongitudinal axis, and a plurality of partition plates positioned in thecasing and preferably sealed thereto to thereby define an intakecollection chamber between a first of the partition plates and the firstend, a discharge collection chamber between a second of the partitionplates and the second end, and a reject collection chamber opposite thesecond partition plate from the second end.

In accordance with one embodiment, the filtration apparatus alsoincludes an intake pipe which is fluidly connected to the intakecollection chamber, a discharge pipe which is fluidly connected to thedischarge collection chamber and a reject pipe which is fluidlyconnected to the reject collection chamber. The intake, discharge andreject pipes may be a single multibore connector which is configured tocouple to a corresponding multibore connector hub which is located,e.g., below the second end of the casing when the filtration apparatusis oriented vertically. This arrangement substantially reduces thefootprint of the filtration apparatus and facilitates the installationand retrieval of the apparatus from a surface vessel.

In accordance with another embodiment, each filtration membrane stackcomprises a number of filtration membranes, each of which is sealed to acorresponding hole in a corresponding partition plate. Each filtrationmembrane comprises an inlet end and an outlet end, and the outlet end isspaced apart from an adjacent partition plate located closer to thesecond end. Thus, the reject fluid from each filtration membrane isallowed to flow into the chamber formed by the two partition platesbefore entering the inlet end of the succeeding filtration membranes. Asa result, if one filtration membrane should become clogged, thatfiltration membrane will not clog the entire filtration membrane stack.

The present disclosure is also directed to a membrane-based waterfiltration system for removing sulfates and other multi-valent ions froma liquid, including but not limited to seawater, groundwater, producedwater, or a mixture thereof. The improved NF membrane disclosed hereinuses a special coating to enhance the separation efficiency byincreasing flux, providing better selectivity and increased rejection,reducing fouling, and increasing tolerance to chemical cleaning. Theimproved NF membrane is particularly useful for removing sulfates fromwater sources for the purpose of water injection, such as in improvedoil recovery (IOR) or enhanced oil recovery (EOR) operations. In suchapplications, the improved water quality results in additional recoveryfrom the reservoir, avoidance of scale in the reservoir and facilities,avoidance of souring due to the proliferation of sulfate-reducingbacteria (SRB's), and better efficiency of the EOR chemicals.

The water filtration system comprising the improved NF membrane of thepresent disclosure is also a superior alternative to existing sulfateremoval systems installed in onshore and offshore facilities andfacilitates installation of a subsea water injection facility. The NFmembrane is suitable for new build systems and for retrofitting existingsulfate removal units (SRU). The application of water treatment systemsincorporating the improved NF membrane will significantly increase theapplicability of such systems on the basis of water quantity/quality,footprint/weight requirements, chemical use and operability, andmarketability in the oil and gas industry.

The water filtration system of the present disclosure includes at leastone NF membrane element which is modified with a super-hydrophiliccoating that can improve NF permeability by at least 25%, as well asimprove foulant repulsion capability and chlorine tolerance, withoutcompromising, and possibly even improving, sulfate rejection. The coatedNF membrane significantly lowers chemical consumption (thus reducingchemical storage footprint) and frequency of cleaning intervention. Thecoating is applicable to various commercially available membranes ofdifferent geometry and materials. In addition, a re-coating system isdesigned to be put into operation (if necessary) when the efficacy ofthe initial coating degrades due to exposure to fouling or cleaningchemicals. The re-coating operation can be implemented afterconventional chemical cleaning is applied. The recoating rig is fullyintegrated into the chemical cleaning system, thus requiring minimaladditional footprint and a simple adjustment to the conventionalchemical cleaning rig. The reduction in chemical consumption due to thereduction in cleaning frequency and the elimination of the need for ade-chlorination step can compensate for the additional footprintallotted for recoating chemicals.

The disclosed water filtration system involves passing the water throughan improved nano-filtration (“NF”) membrane having, preferably, at least25% higher permeability than the prior art, thus enabling applicabilityand ease of deployment for both topside and subsea sulfate removalunits. These improvements are suitable for designing new NF systems andfor retrofitting existing systems to improve productivity. The resultingNF systems require less footprint, which enables their application onspace and weight constrained platforms.

These and other objects and advantages of the present disclosure will bemade apparent from the following detailed description, with reference tothe accompanying drawings. In the drawings, the same reference numbersmay be used to denote similar components in the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevation view of one embodiment of the filtrationapparatus of the present disclosure;

FIG. 2 is an enlarged cross sectional view of the upper end portion ofthe filtration apparatus shown in FIG. 1 ;

FIG. 3 is an enlarged cross sectional view of the lower end portion ofthe filtration apparatus shown in FIG. 1 ; FIG. 4 is a schematicrepresentation of a prior art nano-filtration system;

FIG. 5 is a schematic representation of a first embodiment of thenano-filtration system of the present disclosure; and

FIG. 6 is a schematic representation of a second embodiment of thenano-filtration system of the present disclosure.

DETAILED DESCRIPTION

A general embodiment of the filtration apparatus of the presentdisclosure comprises a tubular casing comprising a longitudinal axis andfirst and second ends, a plurality of elongated filtration membranestacks positioned side-by-side in the casing generally parallel to thelongitudinal axis, and a plurality of partition plates positioned in thecasing and preferably sealed thereto to thereby define an intakecollection chamber between a first of the partition plates and the firstend, a discharge collection chamber between a second of the partitionplates and the second end, and a reject collection chamber opposite thesecond partition plate from the second end. Each filtration membranestack comprises an intake end which is fluidly connected to the intakecollection chamber, a discharge end which is fluidly connected to thereject collection chamber, and a permeate channel which extends betweenthe first and second ends and is fluidly connected to the dischargecollection chamber.

In one embodiment, the filtration apparatus also includes an intake pipehaving a first end which is fluidly connected to the intake collectionchamber and a second end which is fluidly connected to a first connectorlocated proximate the second end, a discharge pipe having a first endwhich is fluidly connected to the discharge collection chamber and asecond end which is fluidly connected to a second connector locatedproximate the first connector, and a reject pipe having a first endwhich is fluidly connected to the reject collection chamber and a secondend which is fluidly connected to a third connector located proximatethe first and second connectors. The first through third connectors canbe separate connectors or a single multibore connector which isconfigured to couple to a corresponding multibore connector hub. Thisarrangement substantially reduces the footprint of the filtrationapparatus and facilitates the installation and retrieval of theapparatus from a surface vessel.

In accordance with an alternative embodiment, each filtration membranestack comprises a number of filtration membranes, each of which issealed to a corresponding hole in a corresponding partition plate. Eachfiltration membrane comprises an inlet end and an outlet end, and theoutlet end is spaced apart from an adjacent partition plate locatedcloser to the second end. Thus, the reject fluid from each filtrationmembrane is allowed to flow into the chamber formed by the two partitionplates before entering the inlet end of the succeeding filtrationmembranes. As a result, if one filtration membrane should becomeclogged, that filtration membrane will not clog the entire filtrationmembrane stack.

An particular embodiment of the filtration apparatus of the presentdisclosure is shown in FIGS. 1-3 . The filtration apparatus, generally10, includes a tubular casing 12 which comprises a first end 14, asecond end 16 and a longitudinal axis 18. As shown in FIG. 1 , thecasing 12 may be oriented generally vertically with the first end 14positioned over the second end 16. The first end 14 may include anopening 20 which is closed and sealed by a cover 22. The cover 22 may beremovably connected to the casing 12 by, e.g., a number of bolts 24. Anupstanding eye bracket 26 may be connected to or formed integrally withthe cover 22 to enable the filtration module 10 to be installed andretrieved from a surface vessel (not shown). As will be discussed inmore detail below, the filtration apparatus 10 may also include amultibore connector hub 28 which is positioned proximate the second end16 of the casing 12.

A plurality of spaced-apart transverse partition plates 30 arepositioned within and preferably also sealed to the casing 12 to therebydivide the interior of the casing into a plurality of longitudinallyspaced chambers, including an intake collection chamber 32 which islocated between a first partition plate 30 a and the first end 14 of thecasing 12, a discharge collection chamber 34 which is located between asecond partition plate 30 b and the second end 16 of the casing, and areject collection chamber 36 which is located opposite the secondpartition plate from the discharge collection chamber. Each partitionplate 30 may be sealed to the casing 12 by, e.g., a suitable ring seal(not shown) positioned between the circumferential edge of the partitionplate and the inner surface of the casing. Alternatively, each partitionplate 30 may be sealed to the casing 12 by being press fit, welded orotherwise attached to the casing. In the example shown in FIGS. 1-3 ,the filtration apparatus 10 comprises five partition plates 30. However,the filtration apparatus 10 may comprise two or more partition plates30, two being the minimum number required to define the intakecollection chamber 32, the discharge collection chamber 34 and thereject collection chamber 36 (which with only two partition plates wouldbe defined between the first partition plate 30 a and the secondpartition plate 30 b).

A plurality of filtration membrane stacks 38 are positioned side-by-sidein the casing 12 generally parallel to the longitudinal axis 18. Eachfiltration membrane stack 38 comprises an intake end 40 which is fluidlyconnected to the intake collection chamber 32, a discharge end 42 whichis fluidly connected to the reject collection chamber 36 and a permeatechannel 44 (FIGS. 2-3 ) which extends axially between the intake anddischarge ends and is fluidly connected to the discharge collectionchamber 34. In the embodiment of the filtration apparatus 10 shown inFIGS. 1-3 , for example, the intake end 40 of each filtration membranestack 38 is sealed to a corresponding hole in the first partition plate30 a, the discharge end 42 of each filtration membrane stack is locatedin the reject collection chamber 36, and the permeate channel 44 of eachfiltration membrane stack is fluidly connected to the dischargecollection chamber 34 by a corresponding discharge tube 46 (FIG. 3 ).

Each filtration membrane stack 38 is comprised of a number of filtrationmembranes 48. The number of filtration membranes 48 in each filtrationmembrane stack 38 will depend on the particular application for whichthe filtration apparatus 10 is designed. In certain applications, eachfiltration membrane stack 38 may be comprised of a single filtrationmembrane 48. In other applications, each filtration membrane stack maycomprise two or more axially aligned filtration membranes 48. In theembodiment shown in FIG. 1 , for example, each filtration membrane stack38 comprises four axially aligned filtration membranes 48.

As seen best in FIGS. 2 and 3 , each filtration membrane 48 comprises aninlet end 50, an outlet end 52 and a permeate passage 54 which extendsaxially between the inlet and outlet ends. In one embodiment of thefiltration apparatus 10, each filtration membrane 48 may comprise across flow filtration membrane. An example of a cross flow filtrationmembrane which is suitable for use in the filtration apparatus 10 is aDOW FILMTEC™ filtration membrane sold by the Dow Chemical Company. Inoperation, a fluid to be filtered enters the filtration membrane 48through the inlet end 50 and is separated into a reject fluid which isdischarged through the outlet end 52 and a permeate fluid which isdischarged through the permeate passage 54. Where each filtrationmembrane stack 38 comprises a single filtration membrane 48, the inletand outlet ends 50, 52 of the filtration membrane define the intake anddischarge ends 40, 42 of the filtration membrane stack, and the permeatepassage 54 defines the permeate channel 44. Where each filtrationmembrane stack 38 comprises two or more filtration membranes 48, theinlet end 50 of the filtration membrane 48 closest to the first end 14of the casing 12 defines the intake end 40 of the filtration membranestack, the outlet end 52 of the filtration membrane closest to thesecond end 16 of the casing defines the discharge end 42 of thefiltration membrane stack, and the permeate passages 54 of the severalfiltration membranes are connected together by suitable means, such astubular connectors 56, to thereby define the permeate channel 44 throughthe filtration membrane stack.

Each filtration membrane 48 is positioned and preferably also sealed ina corresponding hole 58 in a respective partition plate 30. In thismanner, the chambers defined by the partition plates 30 will ideally befluidly isolated from each other. The filtration membranes 48 may besealed to their respective holes 58 by any appropriate means, such aswith a suitable ring seal (not shown) or by being press fit into thehole or welded or otherwise attached to the partition plate 30. Also,the filtration membranes 48 may be positioned within their respectiveholes 58 such that any portion of the filtration membrane from the inletend 50 to the outlet end 52 engages the hole. In the embodiment shown inFIGS. 1-3 , for example, the filtration membranes 48 are positioned suchthat the inlet ends 50 engage the holes 58. When each filtrationmembrane 48 is positioned in its respective partition plate 30 as justdescribed, the inlet end 50 of the filtration membrane is fluidlyconnected to a chamber located on the side of partition plate whichfaces the first end 14 of the casing 12, and the outlet end 52 of thefiltration membrane is fluidly connected to a chamber located on theside of the partition plate which faces the second end 16 of the casing.Thus, each filtration membrane 48 forms a conduit between the successivechambers through which the fluid to be filtered is directed.

In accordance with a non-limiting embodiment of the filtration apparatus10, the outlet end 52 of each filtration membrane 48 is positioned in achamber which is formed between the partition plate 30 in which thefiltration membrane is positioned and the next partition plate closer tothe second end 16 of the casing 12. In other words, the outlet end 52 ofeach filtration membrane 48 is axially spaced apart from the nextpartition plate 30 closer to the second end 16. As a consequence of thisarrangement, the reject fluids which exit the outlet ends 52 of each setof radially aligned filtration membranes 48 are allowed to comminglewithin the chamber before entering the inlet ends 50 of the next set ofradially aligned filtration membranes. In contrast to an arrangement inwhich the outlet end of each filtration membrane feeds directly into theinlet end of the next filtration membrane, allowing the reject fluids toflow through the chamber ensures that a single clogged filtrationmembrane will not render the entire filtration membrane stack 38inoperable. Thus, a single clogged filtration membrane 48 will haveminimal effect of the efficiency of the whole filtration apparatus 10.

Referring specifically to FIGS. 2 and 3 , the filtration apparatus 10also includes an intake pipe 60 having a first end 60 a which isconnected to the intake collection chamber 32 (FIG. 2 ), a dischargepipe 62 having a first end 62 a which is connected to the dischargecollection chamber 34, and a reject pipe 64 having a first end 64 awhich is connected to the reject collection chamber 36. The intake pipe60, the discharge pipe 62 and the reject pipe 64 also compriserespective second ends 60 b, 62 b, 64 b which as shown in FIG. 3 may bepositioned adjacent each other proximate the second end 16 of the casing12, for example below the second end of the casing when the filtrationapparatus 10 is oriented vertically. This arrangement will facilitateconnecting the second ends 60 b, 62 b, 64 b to corresponding fluidconduits of a separate subsea apparatus (not shown) when the filtrationapparatus 10 is installed using, e.g., an ROV or a wireline. The intakepipe 60 and the reject pipe 64 may be configured to extend to theirrespective chambers on the outside of the casing 12 or, as shown inFIGS. 2 and 3 , they may extend axially through the interior of thecasing and through corresponding holes in the partition plates 30.

The second ends 60 b, 62 b, 64 b of the pipes 60, 62, 64 may beconnected to separate fluid connectors (not shown) which are configuredto releasably couple to corresponding fluid connectors on the subseaapparatus or, as shown in FIGS. 1 and 3 , the second ends may beconnected to a single multibore connector, such as the connector 28,which is configured to releasably couple to a corresponding multiboreconnector hub on the subsea apparatus. The multibore connector 28 ispreferably remotely connectable and disconnectable from the separateapparatus to allow for easy installation and retrieval of the filtrationapparatus 10 from a surface vessel.

In operation of the filtration apparatus 10, a fluid to be filtered,such as raw seawater, is conveyed under pressure through the intake pipe60 to the intake collection chamber 32. The seawater enters the inletends 50 of the first set of radially aligned filtration membranes 48,which function to filter certain minerals from the water. The filteredwater migrates to the permeate passages 54 in the filtration membranes48 and is conveyed through the permeate conduits 44 to the dischargecollection chamber 34. The ends of the permeate passages 54 adjacent theintake collection chamber 32 are sealed by plugs 66 to prevent rawseawater from entering the permeate passages in this first set offiltration membranes 48. From the discharge collection chamber 34, thepermeate is discharged to the separate subsea apparatus through thedischarge pipe 60.

At the same time, the unfiltered water, or reject fluid, flows throughthe outlet ends 52 of the filtration membranes 48, into the chamberformed between the first partition plate 30 a and the succeedingpartition plate, and into the inlet ends 50 of the next set of radiallyaligned filtration membranes 48 (or into the reject collection chamber36 if each filtration membrane stack 38 comprises a single filtrationmembrane). The filtration process is then repeated in this and eachsucceeding set of radially aligned filtration membranes 48 until thereject fluid is expelled through the outlet ends 52 of the final set offiltration membranes into the reject collection chamber 36. From thereject collection chamber 36, the reject fluid is discharged to theseparate subsea apparatus through the reject pipe 64.

The present disclosure is also directed to improved filtrationmembranes, specifically nano-filtration (NF) and reverse osmosismembranes, and a water filtration system which incorporates suchmembranes. In one embodiment, the filtration membrane is modified byapplication of one or more layers of a coating, each layer of which isultra-thin, e.g., less than 20 nm, and highly negatively charged and/orsuper-hydrophilic. The coating layers provide an optimal balance betweenhydrophilicity and negative charge density, enabling the modifiedmembrane to exhibit significantly higher water productivity whilemaintaining or increasing membrane selectivity towards sulfates andother multi-valent anions. The coating layers also provide substantialprotection for the membrane from chlorine oxidation and bacterialadhesion.

The chemistry and method of application of the membrane coating aredescribed in U.S. patent application Ser. No. 15/630,792 (now U.S. Pat.No. 10,646,829) by Teledyne Scientific & Imaging, LLC entitled HighFlux, Chlorine Resistant Coating for Sulfate Removal Membranes, whichwas filed on Jun. 22, 2017 and is hereby incorporated herein byreference. The coating is comprised of a material which renders theactive top layer of the membrane hydrophilic. In one embodiment, thecoating may be comprised of two layers applied sequentially. The firstlayer, which is applied to the active top layer of the membrane, may bea thin, conformal hydrophilic layer composed of a combination of ahydrophilic polymer and a surfactant. The combination of the hydrophilicpolymer and surfactant results in an ultrathin but conformal coatingwhich substantially increases the permeability of the membrane. However,this coating may suppress the surface charges of the membrane, which inturn may reduce its sulfate rejection. This effect may be reduced byattaching highly negatively charged molecules in the form of, e.g., anegatively charged material, to the thin first layer of coating toincrease the negative charge density at the surface. This results in asecond layer with high sulfate rejection which does not affect thepermeability of the membrane. The negatively charged molecules can bedyes with sulfonate, carboxyl and other negatively charged groups.

Such a coating has been found to increase the permeability of NFmembranes by greater than 25% without compromising their initial sulfateremoval efficiency. The base uncoated membrane can be made of apolyamide, polyethersulfone or cellulose acetate material. The membranecan comprise spirally wound flat sheets, hollow-fibers, capillary ortubular membranes packaged in cylindrical elements.

In accordance with a more specific embodiment, the coating may comprisea conformal chlorine-resistant, fouling-resistant andpermeate-flux-enhancing coating which is comprised of a hydrophilicpolymer, a surfactant and a charged compound containing one or moresulfonate functionalities and one or more linkable functionalities suchas amine, monochlorotriazine or dichlorotriazine. In this embodiment,the hydrophilic polymer may comprise one or more of the followingmaterials: polydopamine, polyethylene, polyvinyl alcohol, hydroethylcellulose, hydroxyl propyl cellulose, methyl cellose poly(hydroxyethylmethacrylate) and its copolymers, styrene-maleic anhydride copolymer,methyl vinyl ether-maleic anhydride copolymer, polyethylenimine, carboxymethyl cellulose, locust bean gum, bean gum, carrageenan, agar,polyvinylpyrrolidone, sulfonate polysulfone, polyethylene oxide andcopolymers, polyethylene glycol, polyacrylamide, and polysaccharide.

The surfactant may comprise at least one hydrocarbon chain having atleast 16 carbon atoms. More specifically, the surfactant may comprise atleast one hydrocarbon chain having 16 to 24 carbon atoms. For example,the surfactant may comprise N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride. Alternatively, the surfactant may comprise one ormore of the following materials: sodium lauryl sulfate, sodium laurethsulfate, sodium lactate, tetrazolides, phospholipids capable of assuminga zwitterionic state, cocamidopropyl betaine, sulfobetaines andpolyethylene glycol.

Prior to being applied to the filtration membrane, the coating maycomprise a solution having greater than 10 wt % of surfactant. Thecoating may then be applied to a thickness of less than or equal to 20nm, more preferably to less than or equal to 5 nm.

The present disclosure is also directed to a method for applying aconformal chlorine-resistant, fouling-resistant andpermeate-flux-enhancing coating on a filtration membrane. In accordancewith one embodiment, the method involves communicating a firstcomposition comprising a hydrophilic polymer and a surfactant to thesurface of the membrane. The membrane is then exposed to a salinesolution, after which the saline is removed from the membrane, forexample by rinsing the surface of the membrane. Then, a secondcomposition comprising an oxidant dissolved in a buffered solutionadjusted to a pH between 5 and 11 is communicated to the surface of themembrane, after which the membrane is again rinsed. A third compound isthen communicated to the surface of the membrane. In this step, thethird compound is a charged compound containing one or more sulfonatefunctionalities and one or more linkable functionalities such as amine,monochlorotriazine or dichlorotriazine, and this compound is dissolvedin a buffered solution adjusted to a pH between 7 and 11. After the stepof applying the third compound to the first surface, the membrane isonce again rinsed.

Polyamide filtration membranes, which are currently used in a majorityof oil and gas field applications, have very low chlorine tolerance(<1000 ppm-hr) and therefore require a de-chlorination step to preventdegradation of the membranes (see FIG. 4 ). In contrast, the modifiedfiltration membranes of the present disclosure allow the introduction ofa biocide to the water, typically chlorine, which can be generatedin-situ or ex-situ and injected in the pre-treatment system or theeffluent of the pre-treatment system to control the growth ofmicroorganisms in the downstream filtration system. This is due to thefact that the coated membrane has a significantly higher chlorinetolerance, e.g., greater than 5000 ppm-hr, compared to the prior art. Asa result, periodic shock dose injections of chlorine, in addition to acontinuous low dose injection of chlorine to suppress any bio-activity,can be undertaken without the need for a chlorine scavenger upstream ofthe filtration system. In addition, since the injected chlorine is notneutralized before entering the coated membranes, it will serve as botha sterilizing and a cleaning agent, thus minimizing the frequency ofchemical cleaning. This in combination with the elimination of achlorine scavenger application will significantly reduce the chemicalrequirements and hence the chemical cost and storage footprint of thefiltration system. In addition, the inventors have found that as analternative to the use of cleaning chemicals, fouling may be removedfrom the coated filtration membrane using a warm water wash.

One example of a water filtration system which incorporates an improvedNF membrane of the present disclosure is shown in FIG. 5 . Similar tothe prior art embodiment shown in FIG. 4 , the water filtration systemof this embodiment, generally 100, includes a pre-treatment stage 102, aNF stage 104 and a post-treatment stage 106. In this example, seawaterenters the pre-treatment stage 102, where it is subject to a coarsefiltering through, e.g., a screen filter or strainer, possibly followedby a finer filtering through, for instance, a granular media filter or amicrofiltration (MF) or ultrafiltration (UF) membrane. As shown in FIG.5 , the seawater may also be subject to chlorination at thepre-treatment stage 102, for example by being injected with a biocidesolution, such as chlorine containing an oxidizing solution. After thepre-treatment stage 102, the seawater is directed through the NF stage104, where it is separated into a permeate fluid and a reject fluid(which is sometimes referred to as a concentrate). As an example, the NFstage 104 may comprise a filtration apparatus similar to the filtrationapparatus 10 described above, which includes an inlet for the seawater,a permeate outlet for the permeate and a reject outlet for the reject(i.e., the concentrate). The permeate fluid from the NF stage 104 isthen directed to the post-treatment stage 106, where it is subject tothe conventional treatments, such as de-aeration.

For cleaning operations, a portion of the permeate fluid from the NFstage 104 is fed into a mixing tank 108, where it is mixed with one ormore cleaning chemicals 110. This mixture is then injected into theseawater upstream of the NF stage 104 in order to clean the NFmembranes. The cleaning chemicals are then flushed from the NF systemusing, e.g., permeate. If required, the NF membranes may be re-coatedwith coating chemicals 112 in a manner which will be described below inconnection with FIG. 6 . The composition of the coating chemicals may beas described above.

Another example of a water filtration system which incorporates animproved NF membrane of the present disclosure is shown in FIG. 6 . Thefiltration system of this embodiment, generally 200, includes a NFfiltration apparatus 202 comprising a casing or housing 204 within whicha number of NF membranes 206 are positioned. It should be noted that theNF apparatus 202 is depicted generally, and that the actual apparatusmay comprise a plurality of housings which each house one or more NFmembranes. Alternatively, the NF apparatus 202 may be similar to thefiltration apparatus 10 described above. In this example, the housing204 is shown to comprise a seawater inlet 208, a permeate outlet 210,and a reject outlet 212. In operation, seawater from a pre-treatmentstage (not shown) is communicated under pressure through the inlet 208and into the housing 204, where it is separated into a reject fluidwhich is discharged through the reject outlet 212 and an NF permeatefluid which is discharged through the permeate outlet 210. The NFpermeate fluid may then be directed to a permeate tank 214 or conveyeddirectly to a post-treatment stage (not shown).

In this embodiment, the conventional chemical cleaning line has beenmodified to enable introduction of one or more coating chemicals fromtanks 216, 218 into the housing 204 for in-situ periodic replenishmentof the coating layer(s) of the NF membranes 206, if necessary. When theeffectiveness of the initial coating is significantly reduced based onoperational performance (e.g., permeability and solute rejection), thecoating regime may be put into effect. A segment of the NF system, whichmay comprise a single or multiple arrays of housings which each containa number, such as one to eight, of NF membranes 206, is isolated andtaken out of normal operation. Conventional chemical cleaning is thenperformed using cleaning chemicals from a tank 220 in a manner describedfor example in the Dow Filmtec™ fact sheet referenced above, followed bya re-coating regime.

The re-coating regime may involve, e.g., dilution in a mixing tank 222of concentrated coating chemicals from the tanks 216, 218 withde-mineralized water or NF permeate until the desired chemical conditionis satisfied. For this purpose, either the permeate tank 214 or thepermeate outlet 210, or both, may be connected to the mixing tank 222through a permeate line 224. The coating solution is then pumped intothe housing 204 through a service inlet 226 until all the liquidremaining from the chemical cleaning regime has been drained out. If thecoating solution contains particulate components, it may be divertedthrough a filter 228 (e.g., a cartridge filter, MF/UF membrane, etc.)before being fed to the housing 204. Then, the concentrate and NFpermeate are diverted back to the mixing tank 222 through a concentrateline 230, which is connected between the reject outlet 212 and themixing tank, until a steady state is reached, followed by soaking forperiod of time consistent with the coating protocol of the specific typeof coating chemical. The same regime is repeated when multiple steps ofcoating are to be applied. The NF system may then be flushed with NFpermeate from the permeate tank 214 until all residual coating solutionis drained out of the system. Finally, the re-coated array of NFmembranes 206 is put back into normal operation. The coating regime maybe further implemented for the remaining segments of the NF system, ifnecessary. The filtration system 200 may include a number of valves forselectively connecting either the permeate line 224 or the concentrateline 230 to the mixing tank, such as a first valve 232 which ispositioned in the permeate line and a second valve 234 which ispositioned in the concentrate line.

Although the embodiments shown in FIGS. 5 and 6 have been described inconnection with filtration apparatuses comprising NF membranes, thefiltration apparatuses could instead comprise reverse osmosis membranes.

In accordance with another embodiment of the filtration apparatus, themembrane coating is designed such that the rejection of di-valentsulfate ions is not compromised compared to an uncoated NF membrane. Thecoating is therefore particularly suited for desulfation membranes whichare commonly used in Sulfate Removal Units (SRU) that are employed inseawater injection systems to mitigate the risk of souring the reservoirand the risk of plugging the formation due to deposition ofsulfate-based scale.

In accordance with another embodiment of the present disclosure, thepre-treatment stage 102 (FIG. 5 ) may comprise a number of sand-coatedmedia filters which are configured to remove bacteria and suspendedsolids from the feed seawater stream in order to meet the required siltdensity index for nanofiltration.

It should be apparent from the above description that the any of thefiltration apparatuses disclosed herein can be used in a subseaenvironment. As a result, the filtration apparatuses are particularlysuitable for use in injection wells. Locating the filtration apparatuson the seabed allows the treated seawater to be injected straight intothe injection well without the use of conventional topside equipment. Inaddition, the filtration apparatus disclosed herein may be used on thesea floor as a supplementary treatment to the basic treatment regime inwhich solids/fines and bacteria are removed from the seawater.

It should be recognized that, while the present disclosure has beenpresented with reference to certain embodiments, those skilled in theart may develop a wide variation of structural and operational detailswithout departing from the principles of the disclosure. For example,the various elements shown in the different embodiments may be combinedin a manner not illustrated above. Therefore, the following claims areto be construed to cover all equivalents falling within the true scopeand spirit of the disclosure.

What is claimed is:
 1. A filtration apparatus which comprises: a tubularcasing which includes a longitudinal axis and first and second casingends; a plurality of partition plates which are positioned in the casingand are sealed thereto to thereby define a plurality of axiallysuccessive chambers within the casing, including an intake collectionchamber between a first of said partition plates and the first casingend, a discharge collection chamber between a second of said partitionplates and the second casing end, and a reject collection chamberopposite the second partition plate from the second casing end; aplurality of elongated filtration membrane stacks which are positionedside-by-side in the casing generally parallel to the longitudinal axis,each filtration membrane stack comprising an intake end which is fluidlyconnected to the intake collection chamber, a discharge end which isfluidly connected to the reject collection chamber, and a permeatechannel which extends between the intake and discharge ends and isfluidly connected to the discharge collection chamber, wherein an end ofthe permeate channel located adjacent the intake end is sealed from theintake collection chamber; an intake pipe which comprises a first endthat is fluidly connected to the intake collection chamber and a secondend that is fluidly connected to a first connector which is locatedproximate the second casing end; a discharge pipe which comprises afirst end that is fluidly connected to the discharge collection chamberand a second end that is fluidly connected to a second connector whichis located proximate the first connector; and a reject pipe whichcomprises a first end that is fluidly connected to the reject collectionchamber and a second end that is fluidly connected to a third connectorwhich is located proximate the first and second connectors; wherein afluid to be filtered is communicated to the intake collection chamberthrough the intake pipe and is separated by the filtration membranestacks into a permeate which is discharged from the discharge collectionchamber through the discharge pipe and a reject fluid which isdischarged from the reject collection chamber through the reject pipe;and wherein the first, second and third connectors comprise part of asingle multibore subsea connector hub.
 2. The filtration apparatus ofclaim 1, wherein in use the casing is oriented generally vertically withthe first casing end located above the second casing end.
 3. Thefiltration apparatus of claim 2, wherein the intake pipe extends axiallythrough the casing from the second casing end to the intake collectionchamber.
 4. The filtration module of claim 3, wherein the reject pipeextends axially through the casing from the second casing end to thereject collection chamber.
 5. The filtration apparatus of claim 1,wherein each filtration membrane stack is sealed in a corresponding holein the first partition plate.
 6. The filtration apparatus of claim 5,wherein each filtration membrane stack comprises a plurality offiltration membranes; wherein said plurality of filtration membranestacks together define a plurality of axially successive sets ofradially adjacent filtration membranes, and wherein each filtrationmembrane of each of said sets of filtration membranes is sealed to acorresponding hole in a respective one of said_partition plates.
 7. Thefiltration apparatus of claim 6, wherein each filtration membrane ofeach of said sets of filtration membranes comprises: an inlet endthrough which the fluid to be filtered enters the filtration membrane;an outlet end through which the reject fluid exits the filtrationmembrane; and a permeate passage which extends axially between the inletend and the outlet end, the permeate passage being isolated from both anupstream chamber located on a side of said respective one of saidpartition plates facing the first casing end and a downstream chamberlocated on a side of said respective one of said partition plates facingthe second casing end; wherein the inlet end is open to said upstreamchamber; and wherein the outlet end is spaced apart from an adjacentpartition plate located closer to the second casing end such that thereject fluid exits the filtration membrane into said downstream chamber.8. The filtration apparatus of claim 7, wherein each filtration membraneis sealed to the corresponding hole proximate the inlet end.
 9. Thefiltration apparatus of claim 7, wherein the permeate passages of thefiltration membranes in each filtration membrane stack are fluidlyconnected together to thereby form the permeate channel for thatfiltration membrane stack.
 10. The filtration apparatus of claim 9,further comprising a plurality of connector tubes, each of which extendsbetween a corresponding permeate channel and the discharge collectionchamber.
 11. A filtration apparatus which comprises: a tubular casingwhich includes a longitudinal axis and first and second casing ends; aplurality of partition plates which are positioned in the casing and aresealed thereto to thereby define a plurality of axially successivechambers within the casing, including an intake collection chamberbetween a first of said partition plates and the first casing end, adischarge collection chamber between a second of said partition platesand the second casing end, and a reject collection chamber opposite thesecond partition plate from the second casing end; a plurality ofelongated filtration membrane stacks which are positioned side-by-sidein the casing generally parallel to the longitudinal axis, eachfiltration membrane stack comprising an intake end which is fluidlyconnected to the intake collection chamber, a discharge end which isfluidly connected to the reject collection chamber, and a permeatechannel which extends between the intake and discharge ends and isfluidly connected to the discharge collection chamber, wherein an end ofthe permeate channel located adjacent the intake end is sealed from theintake collection chamber; an intake pipe which comprises a first endthat is fluidly connected to the intake collection chamber and a secondend that is fluidly connected to a first connector which is locatedproximate the second casing end; a discharge pipe which comprises afirst end that is fluidly connected to the discharge collection chamberand a second end that is fluidly connected to a second connector whichis located proximate the first connector; and a reject pipe whichcomprises a first end that is fluidly connected to the reject collectionchamber and a second end that is fluidly connected to a third connectorwhich is located proximate the first and second connectors; wherein afluid to be filtered is communicated to the intake collection chamberthrough the intake pipe and is separated by the filtration membranestacks into a permeate which is discharged from the discharge collectionchamber through the discharge pipe and a reject fluid which isdischarged from the reject collection chamber through the reject pipe;and wherein each filtration membrane stack comprises a plurality offiltration membranes; wherein said plurality of filtration membranestacks together define a plurality of axially successive sets ofradially adjacent filtration membranes; and wherein each filtrationmembrane of each of said sets of filtration membranes is sealed to acorresponding hole in a respective one of said partition plates.
 12. Thefiltration apparatus of claim 11, wherein each filtration membrane ofeach of said sets of filtration membranes comprises: an inlet endthrough which the fluid to be filtered enters the filtration membrane;an outlet end through which the reject fluid exits the filtrationmembrane; and a permeate passage which extends axially between the inletend and the outlet end, the permeate passage being isolated from both anupstream chamber located on a side of said respective one of saidpartition plates facing the first casing end and a downstream chamberlocated on a side of said respective one of said partition plates facingthe second casing end; wherein the inlet end is open to said upstreamchamber; and wherein the outlet end is spaced apart from an adjacentpartition plate located closer to the second casing end such that thereject fluid exits the filtration membrane into said downstream chamber.13. The filtration apparatus of claim 12, wherein each filtrationmembrane is sealed to the corresponding hole proximate the inlet end.